Sulfur-carbon nanocomposites and their application as cathode materials in lithium-sulfur batteries

ABSTRACT

The invention is directed in a first aspect to a sulfur-carbon composite material comprising: (i) a bimodal porous carbon component containing therein a first mode of pores which are mesopores, and a second mode of pores which are micropores; and (ii) elemental sulfur contained in at least a portion of said micropores. The invention is also directed to the aforesaid sulfur-carbon composite as a layer on a current collector material; a lithium ion battery containing the sulfur-carbon composite in a cathode therein; as well as a method for preparing the sulfur-composite material.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from U.S. ProvisionalApplication No. 61/239,132, filed on Sep. 2, 2009, the content of whichin its entirety is incorporated herein by reference.

This invention was made with government support under Contract NumberDE-AC05-00OR22725 between the United States Department of Energy andUT-Battelle, LLC. The U.S. government has certain rights in thisinvention.

FIELD OF THE INVENTION

The present invention relates generally to cathode materials for lithiumion batteries, and more particularly, cathode materials forlithium-sulfur batteries.

BACKGROUND OF THE INVENTION

Lithium-ion batteries have found widespread usage as electrical energystorage devices in various portable electronics because of their lightweight relative to other types of batteries. However, particularly forhigh power applications such as electric vehicles, there has been acontinuing effort to improve the energy output and useful lifetime inlithium ion batteries to better suit these high power applications.

Lithium-sulfur (Li/S) batteries, in particular, hold great promise forhigh power applications. Lithium-sulfur batteries have a theoreticalcapacity of 1675 mAhg⁻¹, nearly one magnitude higher than that ofLiFePO₄ (theoretical capacity of 176 mAhg⁻¹). Nevertheless, the Li/Ssystem has not yet been implemented in high power applications becauseof two significant obstacles: the poor electrical conductivity ofelemental sulfur and the intrinsic polysulfide shuttle.

The electrical conductivity of elemental sulfur is as low as 5×10⁻³⁰S/cm at 25° C. Such a low conductivity causes poor electrochemicalcontact of the sulfur and leads to low utilization of active materialsin the cathode. Although compositing elemental sulfur with carbon orconducting polymers significantly improves the electrical conductivityof sulfur-containing cathodes, the porous structure of the cathode stillneeds optimization to facilitate the transport of ions while retainingthe integrity of the cathode after dissolution of sulfur at thedischarge cycle.

The sulfur in the cathode, except at the full charge state, is generallypresent as a solution of polysulfides in the electrolyte. Theconcentration of polysulfide species S_(n) ²⁻ with n greater than 4 atthe cathode is generally higher than that at the anode, and theconcentration of S_(n) ²⁻ with n smaller than 4 is generally higher atthe anode than the cathode. The concentration gradients of thepolysulfide species drive the intrinsic polysulfide shuttle between theelectrodes, and this leads to poor cyclability, high current leakage,and low charge-discharge efficiency.

Most importantly, a portion of the polysulfide is transformed intolithium sulfide, which is deposited on the anode. This depositionprocess occurs in each charge/discharge cycle and eventually leads tothe complete loss of capacity of the sulfur cathode. The deposition oflithium sulfide also leads to an increase of internal cell resistancedue to the insulating nature of lithium sulfide. Progressive increasesin charging voltage and decreases in discharge voltage are commonphenomena in lithium-sulfur batteries because of the increase of cellresistance in consecutive cycles. Hence, the energy efficiency decreaseswith the increase of cycle numbers.

Much research has been conducted to mitigate the negative effect of thepolysulfide shuttle. The bulk of this research has focused on either theprotection of lithium anode or the restraining of the ionic mobility ofthe polysulfide anions. However, protection of the lithium anode leadsto the passivation of the anode, and this in turn causes a slow reactionrate of the anode during the discharge cycle. Therefore, protection ofthe lithium anode leads to the loss of power density. Gel electrolytesand solid electrolytes have also been used as a means for slowing downthe polysulfide shuttle by reducing the ionic mobility of electrolytes.However, the slow transport of ions leads to a low power density.Moreover, neither the protection of lithium anode nor the restraining ofionic mobility completely shuts down the polysulfide shuttle. Althoughthe polysulfide shuttle occurs at slow speed, such modified Li/Sbatteries generally suffer from a significantly shortened lifespan ascompared to lithium ion batteries without these modifications.

Accordingly, there is a need for lithium-sulfur batteries with animproved electrical power output (i.e., improved power density) orimproved usable lifetime. There would be a particular benefit in alithium-sulfur battery possessing both an improved power output and animproved usable lifetime. In achieving the aforementioned goals, thereis a particular need for a lithium-sulfur battery design that minimizesor altogether prevents the irreversible deposition of lithium sulfide onthe lithium anode of the battery.

SUMMARY OF THE INVENTION

In one aspect, the invention is directed to a sulfur-carbon (S/C)composite material useful as a cathodic material in a lithium-sulfurbattery. Special design features have been incorporated into thesulfur-carbon composite material that permits the composite material tosubstantially minimize the formation of lithium sulfide at the anode.

The sulfur-carbon composite material preferably includes (i) a bimodalporous carbon component containing therein a first mode of pores whichare mesopores, and a second mode of pores which are micropores; and (ii)elemental sulfur contained in at least a portion of said micropores. Inthis composite material, the micropores advantageously function asnanosized containers for elemental sulfur, wherein the high surface areaof the micropores provide efficient contact between the transportingelectrons, ions and/or current collector and the insulating sulfur,thereby providing a high electrical conductivity to the compositematerial. In contrast, the mesopores advantageously function tofacilitate transport of lithium ions during the electrochemical cyclingand accommodate the polysulfides and sulfide ions resulting from theelectrochemical reactions. As a result of these dual features, thecomposite material advantageously retains sulfur via the micropores(which minimizes lithium sulfide build up on the anode and which extendsthe useful life of the battery), while at the same time promoting a highenergy output by facilitating lithium ion transport via the mesopores.

In another aspect, the invention is directed to a lithium ion (i.e.,lithium-sulfur) battery containing an anode that contains the abovesulfur-carbon composite material. The lithium battery can employ aliquid, solid, or gel electrolyte medium. In a preferred embodiment, thelithium battery includes a halide-containing additive. Thehalide-containing additive provides the particularly advantageousfeature of reacting with lithium sulfide to produce electrochemicallyreversible polysulfides. The polysulfides can then be further oxidizedto elemental sulfur, thereby providing a regeneration step of sulfur,and thus further extending the useful life of the battery.

In another aspect, the invention is directed to a novel and facilemethod for preparing the sulfur-carbon composites described above. Themethod involves impregnating a porous carbon component with a solutionof elemental sulfur. In a preferred embodiment, the method involves (i)impregnating a bimodal porous carbon component with a solution ofelemental sulfur, wherein the bimodal porous carbon component contains afirst mode of pores which are mesopores, and a second mode of poreswhich are micropores; and (ii) annealing the dried andsulfur-impregnated bimodal porous carbon component under an inertatmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Schematic depicting a general route for producing thesulfur-carbon composites of the invention, as well as the changesoccurring in sulfur distribution and chemistry during discharging andcharging cycles of a battery.

FIG. 2. Graphs showing nitrogen (N₂) adsorption/desorptioncharacteristics of mesoporous carbon and KOH-activated mesoporouscarbon: A) isotherms at 77 K; B) pore size distribution calculated byBarrett-Joyner-Halenda (BJH) method by using the adsorption branch ofisotherm.

FIG. 3. Graph showing thermogravimetric analysis (TGA) results of S/Ccomposites heated under a nitrogen atmosphere at a rate of 10° C./min.

FIG. 4. Graphs showing nitrogen adsorption/desorption characteristics ofS/C composites with various sulfur loading: A) isotherms at 77 K; B)pore size distribution calculated by BJH method by using the adsorptionbranch of isotherm; C) cumulative pore volume; and D) pore volume andsurface area versus sulfur loading. The sample a-MPC is the blankactivated mesoporous carbon without sulfur. The sulfur loading forsamples S_C01 to S-C07 are 11.7, 18.7, 24.8, 30.7, 37.1 45.8, and 51.5wt. %, respectively.

FIG. 5. Graph showing the specific discharge capacity of S/C compositesthat were cycled in 1.0 molal (i.e., 1.0 m) LiTFSI in DOL/DME (55:40) at25° C.

FIG. 6. Current-voltage (I-V) curves of a Li/S battery composed of acathode made from S_C03 S/C composite, an anode of lithium (Li) foil,and an electrolyte of 1.0 LiTFSI in DOL/DME (55:40): (A) dischargingcycles, and (B) charging cycles. The battery was cycled between 1.0 to3.6 volts at 25° C. for 50 cycles.

FIG. 7. Graph showing cyclabilities of Li/S batteries with differentelectrolytes and halide additives: (1) 1.0 m LiTFSI in DOL/DME (55:40),down triangles (2) 1.0 m LiTFSI in DOL/DME (55:40) with 0.5 m LiBr,squares; (3) 1.0 m LiTFSI in DOL/DME (55:40) with 0.05 m LiBr, uptriangles; (4) 1.0 m LiTFSI in DOL/DME (55:40) with saturated LiCl(about 0.15 m), dots; and (5) 1.0 m LiTFSI in TO/DME (55:40) with 0.5 mLiBr, diamonds.

FIG. 8. Cycling current-voltage (I-V) curves of a Li/S battery composedof a cathode made from S_C03 S/C composite, an anode of Li foil, and anelectrolyte of 1.0 m LiTFSI in DOL/DME (55:40) with 0.5 m LiBr as theadditive. (A) discharging cycles, and (B) charging cycles. The cyclingI-V curves shown in this figure were plotted from cycle number 60 to100. The battery was cycled between 1.0 to 3.6 volts at 25° C. for 145cycles.

FIG. 9. Mechanism of the battery chemistry: the left column shows thechemical processes involved in the discharging process, whereas theright column shows the chemical processes involved in the chargingprocess. The shaded blocks show the chemistry without halides. Bromineis meant to be representative of any halide.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, the invention is directed to a sulfur-carboncomposite material (i.e., the “composite material”) containing (i) abimodal porous carbon component containing therein a first mode of poreswhich are mesopores, and a second mode of pores which are micropores;and (ii) elemental sulfur contained in at least a portion of themicropores.

As commonly understood in the art, “mesopores” refer to pore sizes of 2to 50 nm, whereas “micropores” refer to pore size of less than 2 nm. Inone embodiment, the terms “mesopores” and “micropores” are in accordancewith the foregoing definitions. In another embodiment, the term“micropores” can also include pore sizes of less than 3 nm, while“mesopores” are meant to refer to pore sizes of 3 to 50 nm. Generally,micropores and mesopores have a circular shape, which can be anapproximately circular (e.g., ellipsoidal) or completely circular shape.For pores having a circular shape, the pore size refers either to thesurface diameter of the pore (in the case of a completely circular pore)or the longest surface diameter of the pore (in the case of anelliptical pore). The pores can also be non-circular, or evenirregular-shaped. Furthermore, a portion, or even all, of the microporesand/or the mesopores may have one surface dimension within the microporesize range (i.e., <2 nm or <3 nm) or mesopore size range (i.e., 2-50 nmor 3-50 nm), respectively, while another surface dimension is outsideone or both of these ranges. For example, in a particular embodiment,micropores are present in the form of interconnected lines or crackswhich have one surface dimension within a micropore size range andanother dimension in the microscopic or macroscopic range (e.g., morethan 1 micron and up to millimeters or greater) within the bimodalporous carbon material. The microporous interconnected lines or cracksmay, in one embodiment, connect with some or all of the mesopores, oralternatively, not interconnect with mesopores.

The bimodal porous carbon material (and resulting sulfur-carboncomposite material) can be suitably adjusted in pore size (i.e., poresize ranges), pore size distribution (vol % distribution of differentpore sizes or pore size ranges), and other features (e.g., pore wallthickness and pore-pore interspacing or arrangement). For example, inparticular embodiments, the composite material contains microporeswithin a size range having a minimum of 0.5 nm, 1 nm, or 2 nm, and amaximum of 1 nm, 2 nm, or 3 nm. In the same or separate embodiments, thecomposite material contains mesopores within a size range having aminimum and a maximum size selected from any two of the followingvalues: 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm, 25 nm, 30nm, 35 nm, 40 nm, 45 nm, and 50 nm.

At least 5% and no more than 90% of the pore volume of the bimodalporous carbon component is attributable to micropores. In differentembodiments, at least, or no more than, or about 5%, 10%, 15%, 20%, 25%,30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90% ofthe pore volume of the bimodal porous carbon component is attributableto micropores. In other embodiments, the volume percentage attributed tomicropores is within a particular range bounded by any two of theforegoing values (e.g., 10-90%, 10-80%, 20-90%, or 20-80% as particularexamples).

The bimodal porous carbon component can have any suitable total porevolume. For example, in different embodiments, the total pore volume canbe at least 0.5 cm³/g, 0.6 cm³/g, 0.7 cm³/g, 0.8 cm³/g, 0.9 cm³/g, 1cm³/g, 1.1 cm³/g, 1.2 cm³/g, 1.3 cm³/g, 1.4 cm³/g, 1.5 cm³/g, 1.6 cm³/g,1.7 cm³/g, 1.8 cm³/g, 1.9 cm³/g, 2 cm³/g, 2.1 cm³/g, or 2.2 cm³/g, orwithin a range bounded by any two of these values.

The bimodal porous carbon component can have any suitable wall thicknessof the pores. For example, in different embodiments, the wall thicknesscan be about, at least, or less than 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm,7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 15 nm, 18 nm, 20 nm, 25 nm, or 30nm, or a range bounded by any two of these values.

The bimodal porous carbon component can also have any suitable surfacearea. For example, in different embodiments, the surface area can be atleast 300 m²/g, 400 m²/g, 500 m²/g, 600 m²/g, 700 m²/g, 800 m²/g, 900m²/g, 1000 m²/g, 1200 m²/g, 1500 m²/g, 1800 m²/g, 2000 m²/g, 2500 m²/g,3000 m²/g, 3500 m²/g, 4000 m²/g, 4500 m²/g, or 5000 m²/g, or within arange bounded by any two of these values.

The pores can also possess a degree of uniformity. The uniformity can bein any desired property, such as the pore diameter, wall thickness, orinter-pore spacing. Typically, by being substantially uniform is meantthat the pores show no more than 15% or 10%, and more preferably, nomore than 5%, 2%, 1%, 0.5%, or 0.1% deviation in one or more attributesof the pores. In a particular embodiment, the pores possess an orderedspatial arrangement with each other. The ordered arrangement can beeither between micropores, and/or between mesopores, or betweenmicropores and mesopores. In one embodiment, the ordered arrangementincludes at least a partial clustering of micropores and/or mesopores,or a segregation of micropores from mesopores. In another embodiment,the ordered arrangement includes a patterned or symmetrical spatialarrangement of micropores and/or mesopores. The patterned spatialarrangement can be, for example, a hexagonal close packed or cubicarrangement.

The bimodal porous carbon component described herein is preferablyprepared by treating a mesoporous carbon material (e.g., as prepared bythe template methods of the art) with an activation reagent, such aspotassium hydroxide (KOH), O₂, H₂O, CO₂, or ZnCl₂, under elevatedtemperature conditions (e.g., at least 700° C., 750° C., 800° C., or850° C.), following by cooling to about room temperature (e.g., 15-30°C.), and then contact with water. Preferably, the produced carbonmaterial is washed with water followed by treatment with an acidicaqueous solution (e.g., an aqueous solution of a mineral acid in amolarity of at least 0.05, 0.1, 0.5, or 1), and this, preferablyfollowed by washing with deionized water. A particularly preferredmethod of making the bimodal porous carbon material is given in Example1 below.

The micropores and mesopores are generally created by two differentmechanisms, i.e., the micropores are generally created through theactivation process described above, whereas the mesopores are generallycreated by the template process used in preparing the precursormesoporous carbon material. The pore volumes of each set of pores can beindependently adjusted, thereby permitting adjustment of the percentageof micropores with respect to mesopores. For example, the volumecontribution or number of micropores can be adjusted, independently ofmesopore volume contribution or number of mesopores, by adjustingconditions of the activation process, such as by choice of activationreagent, the temperature employed (e.g., from 400 to 1200° C.), and thetime allotted to heating the carbon material at the elevated temperature(i.e., the activation time). Furthermore, as the mesopores can beconveniently prepared by use of a template (e.g., block copolymer,surfactant, silica particle, and polymer particle templates), the size,shape, structure, or other property of the mesopores can be adjusted bysuitable adjustment of the template properties, such as by adjustment oftemplate composition, molecular arrangement, amount of template, andpost-treatment methods of the template.

The pore volumes of micropores and mesopores are generally measured bythe N₂ adsorption method (also known as the Brunauer-Emmett-Teller (BET)measurement). In BET measurements, micropores are filled beforemesopores as nitrogen gas pressure increases. The percentage ofmicropores and mesopores can be calculated based on the adsorbed volumeof nitrogen gas at different pressures.

As used herein, the term “about” generally indicates within ±0.5%, 1%,2%, 5%, or up to ±10% of the indicated value. For example, a pore sizeof about 10 nm generally indicates in its broadest sense 10 nm±10%,which indicates 9.0-11.0 nm. In addition, the term “about” can indicateeither a measurement error (i.e., by limitations in the measurementmethod), or alternatively, a variation or average in a physicalcharacteristic of a group (e.g., a population of pores).

At least a portion of the micropores is occupied by (i.e., “filled with”or “contains”) elemental sulfur. The portion of micropores occupied byelemental sulfur can be, for example, at least 1, 2, 5, 10, 20, 30, or40 volume % (vol %) of the micropores. However, preferably, at least 50vol %, and more preferably, at least 60, 70, 80, 90, or 95 vol % of themicropores is occupied by elemental sulfur. In a preferred embodiment,substantially all of the micropores (e.g., at least 96, 97, 98, or 99vol %) are occupied by elemental sulfur. Preferably, while at least aportion or all of the micropores contain elemental sulfur, at least asignificant portion or all of the mesopores are not occupied by (i.e.,do not contain) elemental sulfur. For example, preferably, no more than50 vol %, and more preferably, no more than 40, 30, 20, 10, 5, 2, or 1vol % of the mesopores is occupied by elemental sulfur.

The amount of sulfur contained in the sulfur-carbon composite (i.e., the“sulfur loading” in terms of weight percentage (wt %) of sulfur by totalweight of the sulfur-carbon composite) depends on the total microporevolume of the composite material. Accordingly, the sulfur loading can beadjusted by suitable adjustment of the total micropore volume. As thetotal micropore volume increases, higher sulfur loadings are madepossible. Thus, by suitable adjustment of the pore volume attributableto micropores, a sulfur loading of, for example, about 5 wt %, 10 wt %,15 wt %, 20 wt %, 25 wt %, 30 wt %, 35 wt %, 40 wt %, 45 wt %, 50 wt %,55 wt %, 60 wt %, 65 wt %, 70 wt %, 75 wt %, 80 wt %, 85 wt % or 90 wt%, can be attained. In other embodiments, the sulfur loading maypreferably be within a range bounded by any two of the foregoingexemplary values (for example, 10-90 wt %).

In another aspect, the invention is directed to a film of the compositematerial described above. For purposes of functioning as a cathodematerial in a lithium battery, the film preferably possesses a thicknessof at least 0.5 μm, and more preferably, at least 1 μm, 2 μm, 5 μm, 10μm, 15 μm, 20 μm, 25 μm, 50 μm, 100 μm, 200 μm, 250 μm, 500 μm, 750 μm,or 1 mm, or within a particular range bounded by any two of theforegoing values.

In another aspect, the invention is directed to a layered materialcontaining a current collector material having coated thereon a layer ofthe sulfur-carbon composite material described above. The layer ofcomposite material can have any suitable thickness, including any of theexemplary thicknesses described above. The current collector materialcan be any conductive material with physical characteristics suitablefor use in lithium-sulfur batteries. Some examples of suitable currentcollector materials include aluminum, nickel, cobalt, copper, zinc,conductive carbon forms, and alloys thereof. The current collector canbe of any suitable shape and have any suitable surface morphology,including microstructural or nanostructural characteristics.

In another aspect, the invention is directed to a lithium-sulfur battery(i.e., “lithium ion battery” or “battery”) which contains theabove-described sulfur-carbon composite material as a cathode component.The lithium-sulfur battery of the invention preferably possesses thecharacteristic of being able to operate for at least 50, 60, 70, 80, 90,100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230,240, or 250 cycles while maintaining a specific discharge capacity(i.e., “discharge capacity” or “capacity”) of at least 350, 400, 450,500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, or 1500 mA-hr/g.

As known in the art, the battery necessarily includes a lithium anode,as well as a lithium-containing electrolyte medium in contact with theanode and cathode. In one embodiment, the lithium-containing electrolytemedium is a liquid. In another embodiment, the lithium-containingelectrolyte medium is a solid. In yet another embodiment, thelithium-containing electrolyte medium is a gel.

Preferably, the electrolyte medium includes a matrix material withinwhich is incorporated one or more lithium ion electrolytes. The lithiumion electrolyte can be any lithium ion electrolyte, and particularly,any of the lithium ion electrolytes known in the art.

In one embodiment, the lithium ion electrolyte is non-carbon-containing(i.e., inorganic). For example, the lithium ion electrolyte can be alithium ion salt of such counteranions as hexachlorophosphate (PCl₆ ⁻),hexafluorophosphate (PF₆ ⁻), perchlorate, chlorate, chlorite,perbromate, bromate, bromite, periodiate, iodate, aluminum fluorides(e.g., AlF₄ ⁻), aluminum chlorides (e.g., Al₂Cl₇ ⁻ and AlCl₄ ⁻),aluminum bromides (e.g., AlBr₄ ⁻), nitrate, nitrite, sulfate, sulfite,phosphate, phosphite, arsenate, hexafluoroarsenate (AsF₆ ⁻), antimonate,hexafluoroantimonate (SbF₆ ⁻), selenate, tellurate, tungstate,molybdate, chromate, silicate, the borates (e.g., borate, diborate,triborate, tetraborate), tetrafluoroborate, anionic borane clusters(e.g., B₁₀H₁₀ ²⁻ and B₁₂H₁₂ ²⁻), perrhenate, permanganate, ruthenate,perruthenate, and the polyoxometallates. Generally, the lithium halidesare not considered as lithium ion electrolytes.

In another embodiment, the lithium ion electrolyte is carbon-containing(i.e., organic). The organic counteranion may, in one embodiment, lackfluorine atoms. For example, the lithium ion electrolyte can be alithium ion salt of such counteranions as carbonate, the carboxylates(e.g., formate, acetate, propionate, butyrate, valerate, lactate,pyruvate, oxalate, malonate, glutarate, adipate, decanoate, and thelike), the sulfonates (e.g., CH₃SO₃ ⁻, CH₃CH₂SO₃ ⁻, CH₃ (CH₂)₂SO₃ ⁻,benzenesulfonate, toluenesulfonate, dodecylbenzenesulfonate, and thelike), the alkoxides (e.g., methoxide, ethoxide, isopropoxide, andphenoxide), the amides (e.g., dimethylamide and diisopropylamide),diketonates (e.g., acetylacetonate), the organoborates (e.g., BR₁R₂R₃R₄⁻, wherein R₁, R₂, R₃, R₄ are typically hydrocarbon groups containing 1to 6 carbon atoms), anionic carborane clusters, the alkylsulfates (e.g.,diethylsulfate), alkylphosphates (e.g., ethylphosphate ordiethylphosphate), dicyanamide (i.e., N(CN)₂ ⁻), and the phosphinates(e.g., bis-(2,4,4-trimethylpentyl)phosphinate). The organic counteranionmay, in another embodiment, include fluorine atoms. For example, thelithium ion electrolyte can be a lithium ion salt of such counteranionsas the fluorosulfonates (e.g., CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, CF₃(CF₂)₂SO₃ ⁻,CHF₂CF₂SO₃ ⁻, and the like), the fluoroalkoxides (e.g., CF₃O⁻, CF₃CH₂O⁻,CF₃CF₂O⁻, and pentafluorophenolate), the fluorocarboxylates (e.g.,trifluoroacetate and pentafluoropropionate), and the fluorosulfonimides(e.g., (CF₃SO₂)₂N⁻).

The lithium ion electrolyte is incorporated in the electrolyte mediumpreferably in an amount which imparts a suitable level of conductivityto the electrolyte medium. The conductivity of the electrolyte medium ispreferably at least 0.01 mS/cm (0.001 S/m) at an operating temperatureof interest, and particularly at a temperature within 20-30° C.

In a preferred embodiment, the electrolyte medium further includes oneor more halide-containing additives (i.e., “halide additives”). Thehalide additive can be any halide-containing ionic compound or material(i.e., a salt). The halide considered herein can be, for example,fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), or iodide (I⁻), or acombination thereof. The countercation can be any inorganic or organiccountercation. The inorganic countercation is typically either an alkali(i.e., Group I) or alkaline earth (i.e., Group II) metal cation.However, boron-group (i.e., Group III), carbon-group (i.e., Group IV,except those halocarbons which contain only a covalent instead of anionic carbon-halogen bond), nitrogen-group (i.e., Group V, except fornitrogen halides), and transition-metal halide compounds are alsoconsidered herein, as long as the halide compound or material is notcorrosive to the lithium anode. It is preferable for the halide additiveto be completely soluble in the matrix material. The halide additive canbe, for example, one or more lithium halides (e.g., LiF, LiCl, LiBr,LiI), sodium halides (e.g., NaF, NaCl, NaBr, NaI), potassium halides(e.g., KF, KCl, KBr, KI), rubidium halides (e.g., RbF, RbCl, RbBr, RbI),magnesium halides (e.g., MgF₂, MgCl₂, MgBr₂, MgI₂), calcium halides(e.g., CaF₂, CaCl₂, CaBr₂, CaI₂), strontium halides (e.g., SrF₂, SrCl₂,SrBr₂, SrI₂), barium halides (e.g., BaF₂, BaCl₂, BaBr₂, BaI₂), Group IIIhalides (e.g., BF₃, BCl₃, AlF₃, AlCl₃, TlF, TlCl, and related compoundsor complexes), Group IV halides (e.g., SiCl₄, SnCl₂, SnCl₄), Group Vhalides (e.g., PCl₃, AsCl₃, SbCl₃, SbCl₅), transition-metal halides(e.g., TiCl₄, ZnCl₂), rare-earth halides (e.g., LaF₃, LaCl₃, CeF₃,CeCl₃), ammonium halides (e.g., NH₄F, NH₄Cl, NH₄Br, NH₄I), alkylammoniumhalides (e.g., MeNH₃Cl, Me₂NH₂Cl, Me₃NHCl, Me₄NCl, Et₄NCl, Bu₄NF,Bu₄NBr, where Me is methyl, Et is ethyl, and Bu is n-butyl), or acombination of any of these. In other embodiments, one or more of theforegoing groups of halide compounds or materials are excluded from theelectrolyte medium.

Preferably, the halide-containing additive is present in the electrolytemedium in at least a trace amount (e.g., at least 0.001 M or 0.001 m,where “M” indicates a molarity concentration and “m” indicates amolality concentration). In different embodiments, the halide additiveis present in a minimum amount of, for example, 0.01 M, 0.05 M, 0.1 M,0.2 M, 0.3 M, 0.4 M, 0.5 M, 0.6 M, 0.7 M, 0.8 M, 0.9 M, 1.0 M, 1.1 M,1.2 M, 1.3 M, 1.4 M, or 1.5 M. In other embodiments, the halide additiveis present in a maximum amount of, for example, 0.5 M, 0.6 M, 0.7 M, 0.8M, 0.9 M, 1.0 M, 1.1 M, 1.2 M, 1.3 M, 1.4 M, 1.5 M, 1.6 M, 1.7 M, 1.8 M,2.0 M, 2.1 M, 2.2 M, 2.3 M, 2.4 M, or 2.5 M. In other embodiments, thehalide additive is present in an amount within a range bounded by anycombination of minimum and maximum values given above, provided that theminimum value is less than the maximum value. Any of the concentrationsgiven above in terms of molarity (M) can alternatively be understood tobe molality (m) concentrations.

In the case of a liquid electrolyte medium, the matrix is a liquid,i.e., composed of one or more solvents. The one or more solvents arepreferably non-reactive with the materials of the anode and the cathode,and furthermore, do not have a deleterious effect on the performancecharacteristics of the lithium ion battery. Preferably, the one or moresolvents are polar aprotic solvents. Some examples of polar aproticsolvents include the nitriles (e.g., acetonitrile, propionitrile),sulfoxides (e.g., dimethylsulfoxide), amides (e.g., dimethylformamide,N,N-dimethylacetamide), organochlorides (e.g., methylene chloride,chloroform, 1,1,-trichloroethane), ketones (e.g., acetone, 2-butanone),dialkylcarbonates (e.g., ethylene carbonate, dimethylcarbonate,diethylcarbonate), organoethers (e.g., diethyl ether, tetrahydrofuran,and dioxane), hexamethylphosphoramide (HMPA), N-methylpyrrolidinone(NMP), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), andpropylene glycol monomethyl ether acetate (PGMEA).

Preferably, in addition to being polar aprotic solvents, the one or moresolvents contain one or more oxyether (i.e., carbon-oxygen-carbon)groups. More preferably, the one or more solvents are ether solvents,i.e., polar aprotic solvents formulated as hydrocarbons except that theycontain one or more carbon-oxygen-carbon groups (e.g., one, two, three,four, five, or six C—O—C groups) in the absence of any other chemicalgroups. The ether solvents typically contain at least three, four, five,six, seven, or eight carbon atoms, and up to nine, ten, eleven, twelve,or higher number of carbon atoms, and can be acyclic or cyclic. Theether solvent may also be saturated, or alternatively, unsaturated(i.e., by the presence of one or more carbon-carbon double or triplebonds).

Some examples of acyclic ether solvents containing one oxygen atominclude diethyl ether, di(n-propyl)ether, diisopropyl ether, diisobutylether, methyl(t-butyl)ether, and anisole. Some examples of acyclic ethersolvents containing two or more oxygen atoms include ethylene glycoldimethyl ether (i.e., dimethoxyethane, or DME, or glyme), diethyleneglycol dimethyl ether (diglyme), triethylene glycol dimethyl ether(triglyme), and tetraethylene glycol dimethyl ether (tetraglyme). Theforegoing exemplary acyclic ether solvents all contain methyl groups asendcapping groups. However, any hydrocarbon endcapping groups aresuitable. Some common endcapping groups aside from methyl groupsinclude, allyl, vinyl, n-propyl, isopropyl, n-butyl, isobutyl,sec-butyl, and t-butyl groups.

Some examples of cyclic ether solvents containing one oxygen atominclude propylene oxide, 2,3-epoxybutane (i.e., 2,3-dimethyloxirane),oxetane, tetrahydrofuran (THF), furan, tetrahydropyran, and pyran. Someexamples of cyclic ether solvents containing two or more oxygen atomsinclude 1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, and thecrown ethers.

In a preferred embodiment, the one or more solvents include at least onesolvent which is an aprotic ether solvent that has a tendency topolymerize, particularly in the presence of a halide (as provided, forexample, when a halide-containing additive is included). Particularlypreferred in this respect are the cyclic ethers, and in particular, oneor a combination of solvents selected from 1,3-dioxolane,dimethoxyethane, and 1,3,5-trioxane. The polymerization of thesesolvents during cycling in the presence of a halide-containing additivehas been found to advantageously improve the cycling performance oflithium-sulfur batteries.

Preferably, the electrolyte medium excludes a protic liquid. Proticliquids are generally reactive with the lithium anode. Some examples ofpolar protic solvents which are preferably excluded include water, thealcohols (e.g., methanol, ethanol, isopropanol, n-butanol, t-butanol,the pentanols, hexanols, octanols, or the like), diols (e.g., ethyleneglycol, diethylene glycol, triethylene glycol), and protic amines (e.g.,ethylenediamine, ethanolamine, diethanolamine, and triethanolamine).

In one embodiment, the electrolyte medium includes a non-polar liquid.Some examples of non-polar liquids include the liquid hydrocarbons, suchas a pentane, hexane, heptane, octane, pentene, hexene, heptene, octene,benzene, toluene, or xylene. In another embodiment, non-polar liquidsare excluded from the electrolyte medium.

The electrolyte medium may also include one or more surfactants. Thesurfactants can be included to, for example, modify or adjust theelectrolyte electron or ion transport properties. The surfactant can beeither an anionic, cationic, or zwitterionic surfactant.

Some examples of anionic surfactants include the fluorinated andnon-fluorinated carboxylates (e.g., perfluorooctanoates,perfluorodecanoates, perfluorotetradecanoates, octanoates, decanoates,tetradecanoates, fatty acid salts), the fluorinated and non-fluorinatedsulfonates (e.g., perfluorooctanesulfonates, perfluorodecanesulfonates,octanesulfonates, decanesulfonates, alkyl benzene sulfonate), and thefluorinated and non-fluorinated sulfate salts (e.g., dodecyl sulfates,lauryl sulfates, sodium lauryl ether sulfate, perfluorododecyl sulfate,and other alkyl and perfluoroalkyl sulfate salts).

The majority of cationic surfactants contain a positively chargednitrogen atom, such as found in the quaternary ammonium surfactants. Aparticular class of cationic surfactants considered herein include thequaternary ammonium surfactants. Some examples of quaternary ammoniumsurfactants include the alkyltrimethylammonium salts,dialkylmethylammonium salts, trialkylmethylammonium salts, andtetraalkylammonium salts, wherein the alkyl group typically possesses atleast 3, 4, 5, or 6 carbon atoms and up to 14, 16, 18, 20, 22, 24, or 26carbon atoms. Another group of cationic surfactants are the pyridiniumsurfactants, such as cetylpyridinium chloride. The counteranions in thecationic surfactants can be, for example, a halide, hydroxide,carboxylate, phosphate, nitrate, or other simple or complex anion.

Some examples of zwitterionic surfactants include the betaines (e.g.,dodecyl betaine, cocamidopropyl betaine) and the glycinates.

In another aspect, the invention is directed to a method for preparingthe sulfur-carbon composite material described above. The methodincludes impregnating a bimodal porous carbon component, having thecharacteristics described above, and as prepared by methods known in theart, or as described herein, with a solution of elemental sulfur. Theelemental sulfur considered herein can be any allotropic form of sulfur.The elemental sulfur considered herein typically consists predominantlyof crown-shaped S₈ molecules. However, numerous other forms andallotropes of sulfur are known, all of which are considered herein. Forexample, by appropriate processing conditions, elemental sulfurcontaining S₆, S₇, S₉, S₁₀, S₁₁, S₁₂, or up to S₁₈ rings, or linear orbranched forms, can be formed. In addition, the sulfur can becrystalline (e.g., of a rhombic or monoclinic space group) or amorphous.The elemental sulfur is dissolved in a solvent to form the solution ofelemental sulfur. The solvent is any solvent capable of dissolvingelemental sulfur to the extent that a solution of, preferably, at least1 wt % (and more preferably, 2, 5, 10, 15, or 20 wt %) sulfur isobtained. Some examples of such solvents include benzene, toluene, andcarbon disulfide.

The driving force which determines the order in which pores are filledis the adsorption energy, which increases with decreasing pore size. Dueto the higher adsorption energy of the micropores as compared to themesopores, the impregnation step described herein first impregnates themicropores with sulfur before the mesopores become impregnated withsulfur. Once the micropores are filled, the small mesopores (i.e., ofabout 3 or 4 microns) will start to fill. Application of a heating(i.e., annealing) step after the impregnation step can further ensurethat the micropores are filled first.

After the bimodal porous carbon component (i.e., “carbon” or “carbonmaterial”) has been impregnated with sulfur, the solvent issubstantially removed from the sulfur-impregnated carbon (i.e., thesulfur-impregnated carbon is dried). By being “substantially removed” ismeant that at least 80%, and more preferably, at least 90%, 95%, or 98%of the solvent is removed. Any method of drying can be used, including,for example, air-drying at ambient temperature (e.g., 15-30° C.),application of a vacuum, and/or heating, e.g., for a suitable period oftime at a temperature of at least 30° C., but no more than 40° C., 50°C., 60° C., 70° C., 80° C., 90° C., or 100° C. After the drying step iscomplete, if desired, another impregnation step can be applied to thedried sulfur-impregnated carbon, followed by another drying step. Anynumber of impregnation-drying cycles can be applied to the carbonmaterial depending on the loading of sulfur desired; i.e., as the numberof impregnation-drying cycles applied to the carbon material isincreased, the sulfur loading increases. By knowing the concentration ofthe sulfur solution and the amount (i.e., mass or volume, asappropriate) of the solution used in each impregnation step, the amountof sulfur impregnated in the carbon material can be calculated bymultiplying the concentration of the solution and the amount of thesolution used. By weighing the carbon material before impregnation withsulfur, the amount of sulfur needed to achieve a particular sulfurloading can also be known.

The impregnation and drying process can be followed by an annealingprocess (i.e., a post-annealing step). Alternatively, the drying stepdescribed above can be omitted, and the impregnation step followeddirectly by an annealing process. The drying process may also bereplaced by an annealing process such that one or moreimpregnation-annealing cycles are applied to the porous carbon material.An annealing process is useful to remove residual amounts of solvents inthe sulfur-impregnated carbon material. The annealing process can alsobe beneficial for filling the pores because sulfur preferably melts atthe annealing temperature. The annealing process is preferably conductedat a temperature above 100° C., and more preferably at least at themelting point of sulfur (e.g., at least 115° C.), and below the boilingpoint of the elemental sulfur used, and more preferably, no more than400° C. For example, in different embodiments, an annealing temperatureof about 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C.,180° C., 190° C., 200° C., 250° C., 300° C., 350° C., or 400° C. isused. Alternatively, the annealing temperature can be within a rangebounded by any two of these values. Preferably, the annealing process isconducted under an inert atmosphere environment. Some examples ofsuitable inert gases include nitrogen and argon.

Examples have been set forth below for the purpose of illustration andto describe certain specific embodiments of the invention. However, thescope of this invention is not to be in any way limited by the examplesset forth herein.

EXAMPLE 1 Preparation of the Bimodal Porous Carbon Material (a-MPC) andTreatment Thereof to Form the Sulfur-Carbon Composite

The general procedure for preparing the sulfur-carbon composite is shownin FIG. 1. FIG. 1 also depicts the change in arrangement of sulfur inthe bimodal carbon material during charging and discharging steps.

Preparation of bimodal porous carbon material (a-MPC). The Precursormesoporous carbon (MPC) was synthesized through a previously reportedsoft-template approach (Wang, X. Q.; Liang, C. D.; Dai, S. Langmuir2008, 24, 7500-7505; Liang, C. D.; Dai, S. Journal of the AmericanChemical Society 2006, 128, 5316-5317; and Liang, C. D.; Hong, K. L.;Guiochon, G. A.; Mays, J. W.; Dai, S. Angewandte Chemie-InternationalEdition 2004, 43, 5785-5789). The MPC has a uniform mesoporedistribution at ca. 7.3 nm, an average wall thickness of about 6 nm, aspecific Brunauer-Emmett-Teller (BET) surface area of 368.5 m²/g, and apore volume of 0.56 cm³/g.

The MPC was treated by KOH activation. In the KOH activation process,the MPC particles (1 gram) were mixed with KOH pellets (4 grams) in anickel crucible with a nickel lid. The crucible was heated to about 800°C. at a ramp rate of 10° C./min in a tube furnace under nitrogen. Thetemperature was maintained at about 800° C. for about 1 hour and thenreduced down to about room temperature. The residue in the crucible waswashed with a copious amount of deionized water and then boiled in 0.1 MHCl for about 30 minutes. The particles were recovered after filtrationand washed to neutral pH with deionized water. The final product ofactivated mesoporous carbon (a-MPC) was dehydrated at about 200° C. forabout 24 hours. After the KOH activation, the BET surface area of theactivated MPC (a-MPC) increased to 1566.1 m²/g; the micropore surfacearea contribution is 962.4 m²/g. The MPC and a-MPC have type IVisotherms with H1 hysteresis (FIG. 2A). The pore size distribution plotin FIG. 2B shows that there is a significant increase of small mesoporesfrom 2 nm to 4 nm, while the large mesopores are slightly enlarged. Themicropore volume of the a-MPC calculated from the thickness-plot (alsoknown as T-plot) is about 0.503 cm³/g .

Preparation of sulfur-carbon (S/C) composite material. Elemental sulfurwas loaded to a-MPC through liquid phase infiltration by using sulfursolution in carbon disulfide (CS₂). The infiltration of sulfur in a-MPCwas carried out through a repetitive solution impregnation/dryingprocedure to attain samples with various loading by using a 10 wt. %sulfur solution in CS₂. The carbon disulfide solvent was evaporated in awell-vented hood. After each impregnation/drying cycle, each sample wasannealed at 140° C. for 1 hour under nitrogen gas. The annealing stepstripped away the residual CS₂. The resulting samples are denoted asS_C01 to S_C07 (which increase in sulfur loading from 11.7 wt. % to 51.5wt. %) as determined by thermogravimetric analysis (TGA) in nitrogen.Shown in FIG. 3 are the TGA curves and derivative weight loss versustime (dw/dt) of the S/C composites. The derivative weight loss versustime indicates the relative evaporation rate of sulfur from the S/Ccomposites.

EXAMPLE 2 Analysis of the Sulfur-Carbon Composite

The surface area (SA) and pore size distribution (PSD) of the S/Ccomposites produced as described in Example 1 were analyzed through N₂adsorption/desorption measurements at 77 K and plotted in FIGS. 4 (A andB). The N₂ isotherms of these samples have a capillary condensation stepcentered at relative pressure (P/P₀) of 0.7 and a H1 type hysteresis.The samples S_C01 to S_C05 have a bimodal pore size distribution withmesopores averaged at 7.3 nm. The cumulative pore volume (FIG. 4C) ofsamples with sulfur loading less than 37.1 wt. % shows an upwardinflection point at pore diameter of 3 nm. Therefore, a portion of thepore volume and surface area of samples with sulfur loading less than37.1 wt. % are attributed to small mesopores (pore size <3 nm) andmicropores. FIG. 4D illustrates the dependence of pore volume andsurface area on sulfur loading. The micropore volume was completelyfilled up when the sulfur loading is above 37.1 wt. %. The isotherm,surface area, and pore size distribution of the sample S_C05 (37.1 wt.%) is close to those of the original mesoporous carbon (MPC) beforeactivation. The sulfur loading of 37.1 wt. % is most likely the criticalpoint for these S/C composites: the micropores and small mesopores (<3nm) in the a-MPC can accommodate up to 37.1 wt. % of elemental sulfur;higher sulfur loading could lead to the occupation of larger mesopores.

EXAMPLE 3 Assembly of Lithium-Sulfur Batteries

The S/C composites, prepared as described above, were pulverized by aball mill and sieved through a 25 μm-opening stainless steel sieve.Slurries were prepared by mixing the S/C composites in a solution of 1wt. % poly(vinylidene fluoride) (PVDF) in anhydrousN-methyl-2-pyrolidinone (NMP) in a 1:5 ratio. The slurries were appliedto 10 mm diameter aluminum current collectors and dried at 120° C. for 4hours. For the purpose of comparison, the original mesoporous carbon(MPC) with 24.1 wt. % sulfur loading and WVA-1500 (MeadWestvacoCorporation) with sulfur loading of 25.2 wt. % were also prepared ascathodes according to the same procedure used for the preparation of S/Ccomposite cathodes. The batteries were assembled as Swagelok cells byusing the S/C composite coated aluminum foil (10 mm diameter, 7 mmthick) as the cathode, and a lithium foil (7 mm thick and 10 mmdiameter) as the anode, a Celgard 3225 separator (10.3 mm diameter), andan organic electrolyte. The organic electrolytes were solutions ofbis(trifluoromethane)sulfonimide lithium (LiTFSI) (99.95% trace metalsbasis) and halides (e.g., LiBr, LiCl, or mixture of these two compounds)in a mixed solvent of 1,3-dioxolane (DOL) and dimethoxyethane (DME) withvolume ratio of 55:40. In addition to the mixture of DOL/DME (55:40),organic solvents, including 1,3-dioxolane (DOL), dimethoxyethane (DME),trioxane (TO), crown ethers, tetrahydrofuran (THF), glymes, andpolyethyleneoxide (PEO), in a broad range of mixing ratios, were alsoinvestigated. The organic electrolyte filled the pores of the cathodeand separator. The cathode, separator, and anode were pressed by aspring to ensure tight contact. A typical cell contained about 1 mg ofS/C composite. No excess of electrolytes were left in the assembledcell.

EXAMPLE 4 Testing of the Lithium-Sulfur Batteries

The batteries were tested in a Maccor 4000 series battery tester. Thebatteries underwent cycling between 1.0 to 3.6 volts. Each cycle wasstarted with the discharge half cycle. Unless specified, all batterieswere tested at the same current of 0.5 mA for both charging anddischarging. The end of charge cycle was determined by one of twoconditions: (1) a charging current lower than 0.05 mA, or (2) a totalcharging capacity greater than 1675 mAh/g, the theoretic maximum of thesulfur cathode. All capacities are normalized by the mass of the sulfur.

The electrochemical performance of the S/C composites with varioussulfur loadings were tested by using 1.0 m LiTFSI in DOL/DME (55:40). Inorder to illustrate the advantageous properties of the S/C compositesprepared from the a-MPC, two additional S/C composites were prepared byusing the MPC that is a material which contains only mesopores, andWVA-1500, an activated microporous carbon which contains mainlymicropores. The surface area of WVA-1500 is 1760 m²/g, which iscomparable to that of the a-MPC. The specific discharge capacities ofthese cathodes were plotted versus cycle numbers in FIG. 5. The S/Ccomposites prepared from mesoporous carbons including a-MPC and MPC hadhigh initial discharge capacities.

The initial discharge capacity decreased with the increase of sulfurloading. When the sulfur loading was 11.7 wt. %, demonstrated by sampleS_C01, the specific capacity of the initial discharge was as high as1584.56 mAh/g, which was about 94.6% of sulfur utilization based on thetheoretical maximum 1675 mAh/g. When the sulfur loading was 51.5 wt. %,i.e. sample S_C07, the initial discharge capacity was 818.22 mAh/g. Itis worth noting that the MPC-supported S/C composite with 24.1 wt. %sulfur had an initial discharge capacity of 1135.87 mAh/g. Of strikingcontrast, the S/C composite prepared from WVA-1500 with 25.2 wt. %sulfur loading displayed a very low initial discharge capacity of only387.64 mAh/g. Therefore, it is evident that the presence of mesoporesaccounts for the high initial discharge capacity. Although theMPC-supported S/C composite had a high initial discharge capacity, ithad a very fast decay of capacity in the following cycles. The capacityof the MPC-supported S/C composite dropped to 163 mAh/g at the sixthcycle. The a-MPC and WVA-1500-supported S/C composites showed highretention of capacities in the cell cycling. The sample S_C01 retained acapacity of 804.94 mAh/g after 30 cycles, and the WVA-1500-supported S/Ccomposites had a capacity of 153.5 mAh/g after 30 cycles, though, asstated above, its initial discharge capacity was only 387.64 mAh/g. Thecomparison of the cycling performances of S/C composites supported byMPC, WVA-1500, and a-MPC demonstrates the following points: (1) themesopores enable the S/C composite to have a high initial dischargecapacity; and (2) the micropores promote retention of cell capacityduring the cycling.

As mentioned earlier, the capacity decay of the Li/S battery is causedby the intrinsic polysulfide shuttle. As long as the concentrationgradient of polysulfide exists in the Li/S battery, the polysulfideshuttle phenomenon remains. The physical adsorption of microporousmaterials such as a-MPC and WVA-1500 can mitigate the migration ofsulfur; nonetheless, the sulfur transport from the cathode to anode isevidenced by the obvious capacity decay shown in FIG. 5 and the cyclingcurrent-voltage (I-V) curves shown in FIG. 6. The cycling I-V curves inFIG. 6 are plots of sample S_C03 cycling for 50 cycles with anelectrolyte of 1.0 M LiTFSI in DOL/DME (55:40). The cell had twodischarge voltage plateaus at 2.2 and 1.9 volts respectively for theinitial discharge. These two plateaus progressively decreased in voltageand duration as the cycles were continued. The two plateaus diminishedto 1.9 and 1.6 volts after 50 cycles (FIG. 6A). A typical charging I-Vcurve of the Li/S battery has a curvy region at average voltage of 2.4volts. The region has a slow increase of voltage during the chargingbecause of the oxidation of polysulfide anions. A rapid increase ofvoltage follows the curvy region at the end of charging due to thedepletion of polysulfide anions. The average voltage and the duration ofthe curvy region reflect the internal resistance and the chargingcapacity of the Li/S battery. The plot in FIG. 6B shows that the averagevoltage of each cycle increases and the duration diminishes when thecycle number increases. Apparently, the cell resistance and capacity ofthe S_C03 cathode decreases with the advancing of cycle numbers. Allthese changes are attributed to the polysulfide shuttle, which causesthe irreversible migration of sulfur from the cathode to the anode.

EXAMPLE 5 Preparation and Testing of Lithium-Sulfur Batteries Containinga Halide Additive

In this experiment, a halide additive was incorporated into theelectrolyte medium in order to improve the cyclability and theutilization of sulfur in Li/S batteries. As further discussed below, thesurprising result was found that the halide additive alters the batterychemistry during the charging and discharging of the Li/S battery,thereby influencing the polysulfide shuttle with the end result ofdriving the migrating sulfur back to the cathode during the chargingcycle.

The sample S_C03 was used as test cathode material for the demonstrationof the effect of halide addition. The cathodes and batteries wereprepared by using the procedure described in the preceding text, exceptthat the electrolyte was prepared as described herein. All cells werecycled between 1.0 to 3.6 volts with a current of 0.5 mA for bothcharging and discharging. The charging cycles were set at the cut-offcurrent of 0.05 mA and cut-off capacity of 1675 mAh/g of sulfur. Thecharging cycles ended when either of these two cut-off conditions wasmet. The cycling capacities were plotted in FIG. 7. A comparativebattery was made by using 1.0 m LiTFSI in DOL/DME (55:40) without addingany halide salt. The test cells were assembled using the followingelectrolytes: (1) 1.0 m LiTFSI in DOL/DME (55:40) with 0.5 m LiBr; (2)1.0 m LiTFSI in DOL/DME (55:40) saturated with LiCl (approximately 0.15m); (3) 1.0 m LiTFSI in DOL/DME (55:40) with 0.05 m LiBr; and (4) 1.0 mLiTFSI in trioxane (TO)/DME (55:40).

The comparative battery had a high discharge capacity in the first 10cycles. The capacity progressively diminished as the cycle numberincreased. The capacity dropped below 400 mAh/g after about 30 cycles.All test batteries had initial discharge capacities over 1300 mAh/g. Thecapacities decreased in the first 10 cycles and then rapidly increasedto over 1100 mAh/g after 15 cycles. The high capacities were maintainedafter the 15^(th) cycle. The cycle life of the battery depended on theelectrolytes and the concentration of the halide. The battery withsaturated LiCl in DOL/DME had the highest capacity, an average over 1600mAh/g for cycles from the 12^(th) to the 60^(th) cycle. The batterysuddenly died after 62 cycles due to a short caused by lithium dendritesthat formed on the anode. The battery with 0.5 m LiBr in DOL/DME had anaverage capacity of over 1300 mAh/g for over 140 cycles. This batteryalso died due to lithium dendrites after 140 cycles. When theconcentration of LiBr was decreased to 0.05 m, the cell died after 20cycles.

Therefore, it is evident that the performance can be optimized bychanging the concentration of halide in the electrolytes. A test batterywith TO/DME showed a capacity over 1100 mAh/g with a cycle life over 210cycles. Several batteries were also tested by using a single solvent,such as DME or triglyme. All cells showed improved cyclability and highutilization of sulfur.

The halide additives made distinct changes to the shapes of the I-Vcurves. Shown in FIGS. 8 A,B are the charging and discharging curves ofthe Li/S battery with 0.5 m LiBr in the electrolyte. These curves wereplotted from the stable cycles. The discharge curves show a plateau at2.4 volts and a shoulder at 1.5 volts. The curves are overlapping witheach other, which indicates a very good cycling stability. The chargingcurves showed two plateaus at 2.6 and 3.2 volts, respectively. Theadditives significantly changed the charging curves by three aspects:(1) the charging voltage did not reach 3.6 volts; (2) the currentremained at 0.5 mA during the entire charging cycle; and (3) the end ofcharging cycle was determined by cut-off capacity. The changes of theI-V curves suggest that the halide participates in the charging anddischarging reaction, and thereby alters the reaction path of the Li/Sbatteries.

A plausible mechanism of the battery chemistry is illustrated in FIG. 9.In a Li/S battery of the art, the discharge typically involves severalsteps: from elemental sulfur to S₈ ²⁻ and S₆ ²⁻; from S₈ ²⁻ and S₆ ²⁻ toS₄ ²⁻ for the high plateau; and from S₄ ²⁻ to S₂ ²⁻ and S²⁻ for the lowplateau. The charging is the reverse of these chemical reactions. Whenthe halides are present in the reaction, the halide can beelectrochemically reduced to elemental halogen or react with sulfur toform polysulfane dihalides. Without being bound by any theory, it isbelieved that the halogens and polysulfane dihalides react with Li₂Sformed on the electrodes to produce electrochemically reversiblepolysulfides. The polysulfides can then be further oxidized to elementalsulfur. Therefore, the halides facilitate the recovery of sulfur whichhad migrated to the anode through the polysulfide shuttle, while alsosubstantially reducing or preventing the production of lithium sulfideat the anode. The recovery of sulfur to the cathode and minimization oflithium sulfide generation at the anode confer a high discharge capacityto the Li/S battery over long cycle periods.

Surprisingly, polymerization of DOL and TO on the cathode was observedwhen halides were added to the electrolytes. The polymer appears topromote retention of a high capacity over long cycle periods. Thus, thepolymerization phenomenon observed on the cathode can be an additionaleffect useful for improving the energy output and usable lifetime oflithium-sulfur batteries.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, those skilled in the art maymake various changes and modifications which remain within the scope ofthe invention defined by the appended claims.

What is claimed is:
 1. A sulfur-carbon composite material useful as acathodic material in a lithium ion battery, the sulfur-carbon compositematerial comprising: (i) a bimodal porous carbon component containingtherein a first mode of pores which are mesopores, and a second mode ofpores which are micropores; and (ii) elemental sulfur contained in atleast 60 vol % of said micropores while no more than 5 vol % of themesopores is occupied by elemental sulfur.
 2. The sulfur-carboncomposite material of claim 1, wherein at least 10% and no more than 90%of the pore volume of the bimodal porous carbon component isattributable to micropores.
 3. The sulfur-carbon composite material ofclaim 1, wherein at least 20% and no more than 90% of the pore volume ofthe bimodal porous carbon component is attributable to micropores. 4.The sulfur-carbon composite material of claim 1, wherein at least 80 vol% of the micropores is occupied by elemental sulfur.
 5. Thesulfur-carbon composite material of claim 1, wherein the micropores arecompletely occupied by elemental sulfur.
 6. A layered material useful asa cathodic material in a lithium ion battery, the layered materialcomprising a current collector material having coated thereon a layer ofa sulfur-carbon composite material comprising: (i) a bimodal porouscarbon component containing therein a first mode of pores which aremesopores, and a second mode of pores which are micropores; and (ii)elemental sulfur contained in at least 60 vol % of said micropores whileno more than 5 vol % of the mesopores is occupied by elemental sulfur.7. A lithium ion battery comprising: (a) a lithium anode (b) a cathodecomprising a sulfur-carbon composite material comprising: (i) a bimodalporous carbon component containing therein a first mode of pores whichare mesopores, and a second mode of pores which are micropores; and (ii)elemental sulfur contained in at least 60 vol % of said micropores whileno more than 5 vol % of the mesopores is occupied by elemental sulfur;and (c) a lithium-containing electrolyte medium in contact with saidanode and cathode.
 8. The lithium ion battery of claim 7, wherein theelectrolyte medium is a liquid.
 9. The lithium ion battery of claim 7,wherein the electrolyte medium is a solid or gel.
 10. The lithium ionbattery of claim 7, wherein the electrolyte medium comprises a matrixmaterial and a lithium ion electrolyte component.
 11. The lithium ionbattery of claim 10, wherein the electrolyte medium further comprises ahalide-containing additive.
 12. The lithium ion battery of claim 11,wherein the halide-containing additive is an inorganic halide salt. 13.The lithium ion battery of claim 12, wherein the inorganic halide saltis an alkali halide metal salt.
 14. The lithium ion battery of claim 11,wherein the halide-containing additive is present in the electrolytemedium in an amount of at least 0.1 m concentration.
 15. The lithium ionbattery of claim 11, wherein the halide-containing additive is presentin the electrolyte medium in an amount of at least 0.5 m concentration.16. The lithium ion battery of claim 10, wherein the matrix material iscomprised of one or more solvents.
 17. The lithium ion battery of claim16, wherein the one or more solvents are polar aprotic solvents.
 18. Thelithium ion battery of claim 16, wherein the one or more solventsinclude one or more oxyether groups.
 19. The lithium ion battery ofclaim 7, wherein the electrolyte medium comprises one or more solvents,a lithium ion electrolyte component, and one or more halide-containingadditives.
 20. The lithium ion battery of claim 19, wherein the one ormore solvents include one or more oxyether groups.